PURPOSE: To reduce a trouble occurring rate as well as to make improvements in safety by constituting a change arm to check movement of a shaft and on-off motion of a jaw when it comes off a changing position in a way of engaging an engager with a cam body inner circumferential surface.

CONSTITUTION: When a change arm 27 is shifted to a turning position from a tool changing position, a tool 5 opens jaws 30 and 31 wide by centrifugal force and gravity, making it so as not to slip off, but at this time an engager 44 is engaged with an inner circumferential surface 42 of a cam body 41 slipped out of a recess 45 so that movement toward these jaws 30 and 31 of a shaft 32 and each on-off motion of these jaws 30 and 31 are checked. In order to check each opening motion of jaws 30, 30 like this, it is ample enough with only the shaft 32 and the cam body 41, so that a structure comes to being simple and a trouble occurring rate is also reduced. And, when the change arm 27 is situated in the turning position, the engager 44 is engaged with the inner circumferential surface 42 of the cam body 41, forcibly closing these jaws 30 and 31, whereby the tool 5 is surely prevented from slipping off, thus safety is improvable.